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Spacing for minimum footcandle (fc) illumination on level ground:

 
Footcandles:
Distance fc Lux
5' 19.4 208
10' 4.86 52
15' 2.16 23
20' 1.22 13
25' 0.78 8
30' 0.54 6
For conversion to lumens, multiply footcandles x 10.